Noise reduction system



F. SHOUP NOISE REDUCTION SYSTEM Aug, 8, 1944;

Filed June 27, 1941 m M M snmu (In/ware 9 Z6 CHOKE wezasa t 3nventor-6'2 f Patented Aug. 8, 1944 UNITED NOISE REDUCTIQN $YSTEM Frederic E Shou p, Haddenfield, N} J., assignorto Radio Corporation of America a.corporation of Delaware Application June 27, 1941, Serial No. 400,003

cclain s. (01. 1250-205).

This invention relates to noise reduction systemsfo'r radio signalamplifiers and the like, which'derive operating current from vibratorpower supply units, as-automobile receivers, for example. and has forits primary'obj'ect to provide an improved and'simplified form; anddistribution, of circuits elements for materially reducing the noisepickup in such apparatus, without materially increasing the cost ofconstruction. It is a further object of this invention, to provide animproved form of high frequency choke coil and circuitdistribution-therefor in connection with automobile radio receiversignal and current supply circuits, which are effective to minimizenoise pickup from the vibratory power supply system thereof.

It is a still further object'of this invention, to provide an improvedarrangement in connection with the power supply system for aradioreceiver of the character refe'rred to, for further effecting 'noisereduction in the receiving circuits'from the vibratory power source. 3

The invention will be further understood from the 7 followingdescription, when considered in connection with the accompanyingdrawing, and

its scopeis pointed out in the appended claims.- In thedrawing, I I

Figure l is a schematic circuit diagram of the signal and power supplysystem of a radio receiver embodyingthe'inventio'n; r Y 1 Figures 2 and3 are external and cross sectional :views, respectively,- andsubstantially full size, iof a "noise'reduction element employed in' thesystem of Fig.1;

Figure 4 is an external view, substantially full size, of another noisereduction element also employed in the system of Fig. 1; and

Figure 5 is a schematic circuit diagram of a modification of the circuitof Fig. 1, showing the signal and power supply'circuits of an automobileradio. receiver embodyingthe invention.

-\ Referring to Figures 1 to 4, inclusive, 5 is an antenna connectionand 6 is a'battery connectionfor supplying signal and power,respectively, to a shielded radio receiver 1 comprising a signal inputcircuit 8, a vibratory power supply device 9,

a power transformer I0 and a power switch I I.

. The lead 6 may be considered to represent the ,high side of thebattery supply circuit for an automobile or other mobile device and issupplied with energy from a battery indicated at I2, having one terminalconnected with the lead 6, while ,the op osite terminalis connectedtoiground or .chassis, as indicated: at I3. The system of the .presentexample, also includes the usual circuits for the ignition systemindicated at: I 4. The latter imparts to thelead 6 a noise signal whichmust be eliminated from the receiving circuits. Likewise, the vibrator 9imparts a noise signal to the distributedwiring of the vehicle and tothe receiving circuits, unless precautions are'taken' to prevent suchpickup.

As is the usual practice, the receiver system is included ina shieldcase asindicated, having a metallic chassis member I6 with the vibrator9 mounted on one side thereof, while the power transformer I0 is mountedon the opposite side as shown. Above the chassis are the input circuit8, and othercircuits of the receiving system which are not shown for thesake of simplifying the drawing. 0

The signalinput circuit 8 is coupled through a transformeril having aprimary winding I8 in the antenna circuit 5 which is extended from theexterioriof the receiver to the interior through a choke coil orinductance indicated at I9 and a supply lead'20 which is bypassed tochassis by suitable. means, such as a capacitor 2|. The cir-' cuit.iscompleted through a lead 22'connected with the chassis or ground, asindicated. The choke coil I9 is close to the casing substantially at theentrance point-of the antenna lead.

On the. power supply side of the receiver, a

.similarchoke coil indicated at 25 is provided directly externally ofthe shield casing I and connected with an interior supply lead 26 whichis also bypassedto chassis or ground. as indicated by the parallelconnected capacitors 21 and 28-, the latter .being a'high frequencybypass for the former. The switch I I is included in the lead-26 whichprovides a connection through. a lead '30 with the power transformer I0.Leads 3I "con- .nect the transformer with the vibrator, asindicated;which serves to interrupt the primary current with a-full waveaction inthe present an ample. .As the form of vibrator and transformer do notconcern the invention, further description .is believed to'beunnecessary.

However, it will be noted thatthe'transformer is included in a metallicshield casing 33and that I0- is mounted belowthe from the input andother circuits of the receiver,

the vibrator 9 is mounted above the 'chassis'for convenience inservicing and isremovable from the apparatus by the usual contact meansindi:- 'cated at 34. In engaging the-contactsthevibrator is caused toseat within a metallic ring member or short-circuited conductive turn 35which overlaps the joint between the vibrator and chassis sufficientlyto prevent magnetic leakage from the vibrator circuits to the inputcircuit 8 and other circuits (not shown) of the receivin system.

It has been found that this ring must be substantially integral with orwelded or otherwise s'ecured to the chassis at a plurality ofpoints inorder that the noise pickup from the vibrator may.

elfectively be eliminated.

The casing of the vibrator 9 is grounded to the cup or ring 35 at one ormore points, as indicated by the connections 36 to provide electrostaticshielding. This may be provided by any suitable means providingconnections as shown between the vibrator casing and the ring 35 and atleast one such connection should be provided in each case. It will alsobe noted that the filament connection for the receiving system is takenfrom the lead 30 through 'a'suitable high frequency choke coil 31. Thismay be of any suitable construction provided it is relatively small.

It has-:also been found that the choke coils l9 and 25 must be ofrelatively small diameter with respect to their length, havingsubstantially the shape and size shown in Figs. 2, 3 and 4.

As shown in Fig. 3, the choke coil may comprise a flexible core 40 ofinsulating material and of substantially minimum diameterfor a givenwire size for the coil or winding 4| thereon. The latter comprises asingle layer solenoid of insulated conductor with the ends connected tometallic end ferrules 42 which in turn are connected with flexiblesoldering leads orwire 43 at the ends of the unit. The winding maycomprise several hundred turns of No. 40, B; & S. gauge, enameled copperwire, for example, and is covered preferably with a flexible insulatingbraid, indicated at 44.

In the case of the coil 25, it is similarly constructed of insulatedwire 46 and is covered by an outer insulating braid 41. It will be notedthat this wire is relatively heavy for carrying the 1 I battery currentto the vibrator and other circuits of the receiver. By way of example,No. 18 B. and S. gauge enameled wire had been used and may comprise aplurality of turns, of the order of 20, for example, in present case;

This system and type of choke coil serves to minimize undesiredelectrical noise impulses caused by the ignition system and vibrator 9'in the supply circuits entering the sensitive receiving circuits in amore effective'manner than any other known arrangement. It has madepossible a greater sensitivity with substantially no noise interferencein present commercial installations. Since the coils are relativelysmall, they may readily be inserted in the supply circuits at anydesired points. As shown, these coils are preferably external to thereceiver and shield housing at the points of entry of the supply leadsthereto. In this case, the capacitance of the connectors and cableclamps with the inductances l9 and 25, provide elfective filters withoutadditional capacitors, which resonate in the range of noise-pulses, forexample, between 7.5 and 75 m. c. The external capacitance is indicatedin dotted lines in Fig. 1 at 48 and 49 and may-be. of the order of 40mmfd. in some cases. I

Although filter choke coils have been used heretofore for the reductionof undesired signal impulses, it has-been found that coils of unusuallysmall diameter relative to the length, for exam- 1 same relative lowcost.

'tice range in length from one inch to two-andone-half inches dependingupon the extent or amplitude of the noise impulses to be eliminated. Thediameter 'of the internal core here considered, is off the order ofone-sixteenth inch when starting the winding process, and depending upon.the nature of the core material, this may be reduced in various degreeswhen the coil is completed. The inner core may be a braid or cord of anysuitable insulating material and the outer cover may be of any suitableinsulating or protectingmaterial such as cotton braid, insulatedmetallic shielding molded rubber, or so-called spaghetti insulation. 1

It has been customary to ground vibrators and casings with aspring-fingered cup. This, however, has proved to be not eifective inpreventing electrical disturbances from entering the receiving circuits.The use of a short circuited ring shield about the vibrator basesubstantially integral with the chassis minimizes noise pickup from thesources referred to, even though slightly spaced about the vibratorbase. The cup or ring thus secured to or formed with the chassis,becomes a grounded short-circuited turn or winding for theelectromagnetic field of the vibrator where the electrical connectionsare made below the casing. In certain apparatus, this ring may be formedfrom the chassis material. In any case there must be no magnetic fluxleakage between the ring and the chassis aboutthe power supplyconnection to the vibrator device.

Referring now to Fig. 5, the chassis of a radio receiver is indicated at50 and is connected to ground as indicated at 5|. This serves as ashield between radio receiving circuits. above and power supply circuitsbelow, the latter comprising a vibrator device 52, the power transformer53 and A battery supply .lead 54 connected to the transformer and to afilament circuit 55 through an external inputchoke coil 56 and suitableinternal choke coils indicated at 51 and 58, the latter being an ironcore choke coil, provided with a bypass capacitor 59 to ground. i

The distributed capacity of the supply lead external to the receiver isindicated at 60. With the chokecoil 56 this provides an effectiveexternal filter for removing from the receiving circuits, radiation backfrom the A" battery lead 54 to the vehicle or other power supply meanswith which the system is used. Internal bypass capacitors are indicatedat 62 and 63. providing low and high frequency .bypass means for the Aby s pply. A Above the chassis, the antenna lead, indicated at 65,isconnected through a coupling capacitor 66 with an input primaryWinding 61 coupled to .a, secondary winding -68 connected through achoke coil 69 and a bypass capacitor 10 with ground; The choke coil.69may be located internally of the receiver shield casing indicated by thedotted outline l2, withoutinterfering with the operation of the noisereduction system. This is for the reason that. thechoke coil is of suchrelatively small diameter that it sets up a relatively small field forcoupling with other elements of the receiver, such as the input circuitindicated at 13 for the first amplifier tube 14 and the output circuitsl and 16.

The distributed capacity of the antenna input lead is indicated at 18.With the choke coil 69, this provides a filter for the undesiredignition and other noise impulses received on the antenna lead.The-operation of the circuit is otherwise the same as that described inconnection with Fig. 1, and the choke coils 69 and 56 correspondrespectively with the construction shown in Figs. 2-3 and 4. 1

While the invention has been described in connection with automobileradio receiving systems, it is not limited thereto, but may be providedin connection with any receiving system haying noise impulse sourcesassociated therewith such as vibratory power supply and ignitioncircuits.

I claim as my invention:

1. The combination with shielded signal amplityin apparatus, of externalsignal and power supply leads extending into said apparatus, flexibleinductive filter elements adjacent to ,the points of entry connectedserially in said firstand second named leads, providing with thedistributed capacity thereof with respect to said shield and said powersupply and leads high frequency noise filter for said leads, saidinductive elements each comprising a single layer conductive windinghaving an internal diameter of the order of one-sixteenth of an inch,and having a length at least of the order of one inch.

2. In a radio receiving system, the combination with a signal receiverhaving a shield fcasing, a chassis and signal and power supply leads forsaid receiver, of an unshielded elongated small diameter flexible chokecoil in each of said leads external of and adjacent to said casingproviding filtering elements for said leads to prevent the entry of highfrequency disturbances into said receiver, the internal diameter of saidchoke coils being of the order of a small fractional part of an inch andsubstantially a minimum consistent with effecting a single layer windingwith a relatively small wire gauge, and having a length to diameterratio greater than 10:1, and capacity means for causing said choke coilsto resonate in a relatively high frequenc disturbance range.

3. The combination with signal amplifying apparatus, of a supplylead'therefor, a high frequency noise filter in said lead adjacent tosaid apparatus comprising the distributed capacity of said lead and anunshielded elongated single layer flexible inductance winding comprisinga plurality of conductive turns of relatively small gauge wire having aninternal diameter of the order of one-sixteenth of an inch and a lengthof the order of at least one inch.

4. A choke coil element for a high frequency noise filter system,comprising an unshielded elongated flexible inductance winding ofrelatively small gauge wire having an internal diameter of the order ofa small fractional part of an inch, and a length to diameter ratio inexcess of 10:1.

5. A choke coil element for a high frequency noise filter system,comprising an unshielded elongated flexible inductance winding ofrelatively small gauge wire having an internal diameter of the order ofa small fractional part of an inch, and a length to diameter ratio inexcess of 10:1, and means providing spaced terminal ends for saidwinding and aprotective covering for said winding interposedbetween saidterminal ends.

6. A choke coil element for a high frequency noise filter system,comprising an unshielded elongated inductance winding of relativelysmall gauge wire having an internal diameter of the order of a smallfractional part of an inch, and a length to diameter ratio in excess of10:1, means providing a-fiexible core element for said Winding, a pairof spaced terminal ends for said choke coil element to which saidWinding is connected, and means providing a protective covering for saidwinding between said terminal ends.

FREDERIC F. SHOUP.

